Method of determining percent solids in a saturated solution



Feb. 13, 1968 G. w. BARNETT 3,368,389

METHOD OF DETERMINING PERCENT SOLIDS IN A SATURATED SOLUTION OriginalFiled July 24, 1961 2 Sheets-Sheet 1 ll 29 q L DETECTOR Inventor Glem'oy W game Attorney Feb. 13, 1968 G. w; BARNETT 3,368,389

METHOD OF DETERMINING PERCENT SOLIDS IN A SATURATED SOLUTION OriginalFiled July 24, 1961 2 Sheets-Sheet DENSITY-'- PERCENT CRYSTALS InveniorGlenroy W. IBarneTT Attorney United States Patent Ot'tice 3,368,389Patented Feb. 13, 1968 3,368,389 METHUD F DETERMINING PERCENT SOLIDS INA SATURATED SQLUTION Glenroy W. Barnett, Dublin, Ohio, assignor toIndustrial Nucleonics Corporation, a corporation of Ohio Continuation ofapplication Ser. No. 126,155, July 24, 1961. This application May 28,1964, Ser. No. 371,833 8 flaims. (Ci. 73-53) This is a continuation ofmy copending application Ser. No. 126,155, filed July 24, 1961, forMeasuring System, now abandoned.

This invention relates generally to a method for performing densitymeasurements and more particularly it relates to a penetration radiationdensity gauge for determining the percent of solids in solid form in aliquid in which the solid is soluble.

In many chemical processes a solution of some soluble material in asolvent liquid is processed through a stage in which the solids insolution crystallize out of the solution and are present as solidparticles. For example, in a saturated solution of a salt or othercompound, the concentration of the solution by boiling or other meansbeyond the saturation point results in crystals forming as the solidmaterial in solution crystalizes out to maintain the liquid solution insaturation. If the density of such a mixture of solution and crystals ismeasured using a conventional radiation density gauge device, a readingwill be obtained which indicates the composite eifect of the density ofthe solution containing dissolved solid material and the presence of acertain percentage by weight in the liquid of solid crystallinematerial. Whenever it is necessary to measure the actual percent ofcrystals present in a mixture such density measurements are of littlevalue. Prior arrangements for measuring the percent of solids in such aliquid carrier have required two density measurements one of the densityof the composite mixture of crystalline solid carried in the saturatedsolution, as previously described, and a second measurement made of thesolution alone after the solid material has been removed by filtering orother means. The difference in these two density measurements could bepresented on an indicator calibrated to show percent solids. Thisarrangement has obvious disadvantages such as requiring two densitymeasuring devices as well as the fact that the final result is notobtained from the original liquid but only after the liquid has beenprocessed to remove the solids therein.

The difficulty in achieving the true measure of the percent solidspresent in the mixture derives from the fact that the solubility of thesolid in the liquid varies with temperature and hence at differenttempertaures the liquid itself with the solid in solution therein willhave a varying density. Hence the density of the solution will changefor different temperatures and a constant correction cannot be made tothe density measurement of the mixture of solution and solids.

The present invention overcomes this difliculty by providing an improvedmeasurement method for making the measurement which compensates theindication obtained in accordance with the temperature of the solutionas established by previous measurements thereof, thereby giving thedensity of saturated solutions as a function of temperature. Thisarrangement permits a single density measurement of a solutioncontaining crystallized solids and a measurement of the temperature ofsuch a solution to produce an indication which can be accuratelycalibrated in percent solids irrespective of the temperature at whichthe measurements are conducted.

It is acordingly the primary object of the present invention to providean improved method for measuring the relative concentration of solids ina saturated solution of the solid in a liquid.

A further object of the invention is to provide an improved compensatingcircuit for modifying the indication obtained from a density gauge inaccordance with temperature to indicate percent solids in a liquid.

These and other objects of the invention will be apparent from thefollowing detailed description taken in conjunction with theaccompanying drawings wherein:

FIG. 1 is a schematic representation of a measurement system employingthe invention;

FIG. 2 is a schematic wiring diagram of one form of compensatedmeasuring circuit;

FIG. 3 is a schematic wiring diagram of a preferred form of measuringcircuit; and

FIG. 4 is a family of curves showing the relationship of the variablesinvolved for one particular salt solution Referring now to FIG. 1 theinvention may be applied to any process involving a liquid 11 which maybe contained in a suitable container 12 or be passing through a processstep in which it is arranged to flow into the container 12 through aninlet passage 13 and an outlet passage 14. The solution 11 upon reachingsaturation crystallizes out solid particles 15 the measurement of whichin terms of percentage in a liquid is accomplished by the presentinvention.

The single density measurement performed in accordance with theinvention may use conventional radiation density gauge techniques whichemploy a source 16 of gamma rays. X-rays, or other suitable radiantenergy. The rays which pass through the solution 11 and the particles 15contained therein are detected by a detector 17 in conventional mannerto produce a signal in accordance with the absorption in the pathbetween the source 16 and detector 17 and hence representative of thedensity of the material in that path. Any suitable density measuringapparatus may be employed for this purpose and the various arrangementsof measurement circuits and disposition of the source and detector onopposite'sides of the material path may be in accordance with standardpractice in the density measuring art. The signal from the detector 17is applied to an amplifier device 18 to produce an indication on anindicator 29 in accordance with the signals detected by the detector 17.

In accordance with the invention a temperature sensor 21 is provided tosense the temperature of the solution 11 and produce a change in anelectrical quantity which can be used in the detector amplifier 18 tomodify the reading on the indicator 29. The device 21 may be of anysuitable character but conventionally takes the form of a thermistorwhich is a resistance element having a resistance characteristic thatvaries in accordance with the temperature. Such elements may either havepositive or negative temperature coeflicients and the choice of theparticular element will be determined by the circuit in which thethermistor is placed to compensate the indication obtained.

If desired the compensated signal applied to indicator 29 can also beapplied as an input to an automatic controller 20 which may be arran edto operate in known manner to control some variable of the process suchas the temperature of the solution 11. For this purpose the controller20 is shown operating to control a heater.1 0 to maintain apredetermined condition of the liquid v11..

Referring now to FIG. 2, one form of measurement circuit is shown inwhich an electrometer tube 22 has a grid input signal from terminal 23derived from the detector 17. The output from the plate electrode of thetube 22 is directly coupled to the grid of a cathode follower 24. Thecathode circuits of the tubes 22 and 24 have resistors 25, 26, 27 and 28which form a bridge circuit between which a meter 29 is connected withan appropriate multiplier 31. The meter 29 indicates the quantitymeasured by the circuit.

This arrangement of FIG. 2 ordinarily has a bucking circuit associatedtherewith including the tube 33 whereby a signal in opposition to thatat input electrode 23 is provided from a potentiometer 34 in the platecircuit of tube 33. The tube 33 normally has a fixed grid potential andthe adjustment of the tap on the potentiometer 34 provides a zeroingadjusting for the meter 23. In order to compensate for temperaturevariations in the solution, for the reasons outlined hereinabove, thecathode circuit of the tube 33 may include a thermistor 35 which isphysically located in the temperature sensor probe 21, FIG. 1.Thermistor 35 varies the overall cathode resistance and hence theoperating bias on the tube 35 by an amount sufficient to compensate forchanges in the indication on meter 29 resulting from temperature changessensed by the-thermistor.

In the circuit of FIG. 2 the resistor 35 is required to be a positivetemperature coefficient resistor having sufficient slope for itsresistance-temperature characteristic to produce the desired change inthe indication of meter 29. Since positive temperature coefficientresistors are not as readily obtainable as negative coeflicientresistors an alternative preferred circuit is shown in FIG. 3.

The circuit of FIG. 3 is generally similar to that of FIG. 2 andcorresponding parts have been given like reference numbers throughout.The changes which have been made are in the circuit of the compensatingtube 33 wherein a fixed cathode resistor 36 is provided having asubstantially Zero temperature coefiicient as do the other resistors inthe circuit except those specifically so designated as having a non-zerotemperature coeflicient. To obtain the compensation required, the gridcircuit of the tube 33 in FIG. 3 compromises a voltage divider 38, 39,40 connected between the positive and negative D.C. supply. The grid oftube 33 is connected to an intermediate point on the voltage divider.The resistor 38 is shunted by a negative temperature coefiicientthermistor 37. The thermistor 37 is preferably enclosed in a suitablehousing to be placed directly in the environment the temperature ofwhich is to be moniiored. Thus the thermistor 37 will be located as thetemperature sensor 21 in FIG. 1 for responding to the temperature of theliquid 11. Since the resistor 37 is in the grid circuit of the tube 33the overall effect at the tap of potentiomenter 34 will be opposite tothat which would be produced with the same temperature coefiicientresistor in the cathode circuit. In addition, the gain of the tube 33 iseffective to amplify the change produced in the thermistor 37 due totemperature and accordingly a suitable correction factor for suchchanges can be selected at the tap of the potentiometer 34 for modifyingby the necessary amount the indication given by the meter 29.

The amount by which the indication on meter 29 is modified by thethermistor 37 will of necessity have to be determined for eachparticular substance which is to be measured, For this purpose therelation between the specific gravity of the saturated solutioncontaining the crystals, the percent of crystal solids in the mixtureand the temperature of the mixture must be determined. Once thisrelation is determined the appropriate temperature coefiicient for theresistor 37 can be selected and the instrument calibrated to produce thedesired result.

As a specific example of the relation between the aforementionedvariables FIG. 4 shows a family of curves in which density is plotted asa function of percent crystals in the solution with temperature as aparameter. Thus for any density and any temperature the weight of thecrystals in percent can be determined. Since the density gauge producesa signal proportional to the density of the solution, and since thetemperature is sensed in accordance with the invention by the sensor 21,two of the variables of FIG. 4 are present as inputs to the measuringsystem and accordingly the indication produced on meter 29 can becalibrated to read directly in percent by weight of the crystalstherein.

As an example of the utility of the present invention the followingspecific values will be considered for the saturated solution of NaNOplotted in FIG. 4. In a process which normally operates at 60 C. with aconcentration of 7% crystals a density reading of 1.48 would be obtainedfrom a normal density gauge and this input to the circuit of the presentinvention along with the 60 C. environment for the temperature sensorwould be calibrated to produce the reading of 7% crystals. If thedensity were to increase to 1.51 without a change in temperature thecorresponding point on the 60 C. curve of FIG. 4 would indicate that thecrystal concentration had increased to 12%. If, however, the densityremained at 1.48 so that no change in the signal input from the detector17 occurs when there is no change in density, this change from 7% to 12%must be produced as an indication on the meter 29 by the thermistor inthe circuit such as the thermistor 37 in the circuit of FIG. 3. Anappropriate slope for the temperature coefficient characteristic of thethermistor 37 can be selected to produce this change. Since therelations among the variables are substantially linear, the calibrationof the system can be readily effected for a substantial range of values.In any instances where the relation between the variables involved isnot linear over the operating range of the process being measuredsuitably shaped electrical compensation characteristics can be providedas a function of temperature by various means known in the art.

Although a particular arrangement for producing a temperaturecompensated measurement as applied to a specific process has beendescribed, it will be appreciated that the invention is capable of broadapplication wherever variations in temperature produce a change inspecific gravity of a solution which, using the prior art measurements,could not be readily distinguished from the change in specific gravityresulting from the change in the actual solids present in the solution.Accordingly the invention is to be considered to include the variousarrangements which can be devised for achieving this result which maydiffer considerably from that of the disclosed apparatus embodiment andalso to include the method when performed by equivalent apparatus ornon-equivalent apparatus.

I claim:

1. The method of determining in a variable temperature environment theconcentration of solids in a mixture of said solids and a saturatedsolution of said solids, the density of said saturated solutionincreasing with the temperature of said mixture, said method comprisingthe steps of:

passing a beam of radiation through said mixture,

detecting said radiation passing through said mixture to provide a firstsignal having a first component that is a function of said solidsconcentration and a second component that is a function of saidsaturation solution density,

sensing the temperature of said mixture to provide a second signal thatis a function of the density of said mixture,

and combining said second signal with said first signal to provide athird signal that is a function only of the concentration of said solidsin said mixture.

2. The method of regulating the concentration of solids in a mixture ofsaid solids and a saturated solution of said solids, the density of saidsaturated solution increasing with the temperature of said mixture, saidmethod comprising the steps of:

passing a beam of radiation through said mixture,

detecting said radiation passing through said mixture to provide a firstsignal having a first component that is a function of said solidsconcentration and a second component that is a function of saidsaturation solution density.

sensing the temperature of said mixture to provide a second signal thatis a function of the density of said mixture,

combining said second signal with said first signal to provide a thirdsignal that is a function only of the concentration of said solids insaid mixture,

and controlling the temperature of said mixture in accordance with saidthird signal to maintain a predetermined condition of said mixture.

3. The method of determining in a variable temperature environment theconcentration of salt crystals in a mixture of said crystals and asaturated solution of said salt, the density of said saturated solutionincreasing with the temperature of said mixture, said method comprising:

passing a beam of radiation through said mixture,

detecting said radiation passing through said mixture to provide a firstsignal having a first component proportional to said salt crystalsconcentration and a second component proportional to said saturationsolution density,

sensing the temperature of said mixture to provide a second signalproportional to the density of said mixture,

and combining said second signal with said first signal to provide athird signal proportional only to the concentration of said saltcrystals in said mixture.

4. The method of regulating the concentration of salt crystals in amixture of said crystals and a saturated solution of said salt crystals,the density of said saturated solution increasing with the temperatureof said mixture, said method comprising the steps of:

passing a beam of radiation through said mixture,

detecting said radiation passing through said mixture to provide a firstsignal having a first component proportional to said salt crystalsconcentration and a second component proportional to said saturationsolution density,

sensing the temperature of said mixture to provide a second signalproportional to the density of said mixture, and

combining said second signal with said first signal to provide a thirdsignal proportional only to the concentration of said salt crystals insaid mixture,

and controlling the temperature of said mixture in accordance with saidthird signal to maintain said crystal concentration substantiallyconstant.

5. The method of determining in a variable temperature environment thesolids concentration of a mixture ofv said solids in a saturatedsolution of said solids, the density of said mixture being a function ofthe density of said saturated solution and the concentration of saidsolids in said mixture, said method comprising the steps of:

measuring the density of said mixture,

sensing the temperature of said saturated solution to measure thedensity thereof, and

combining said density measurements to provide an indication of saidsolids concentration.

-6. The method of determining in a variable temperature environment thesolids concentration of a mixture of said solids in a saturated solutionof said solids, the density of said mixture being a function of thedensity of said saturated solution and the concentration of said solidsin said mxiture, said method comprising the steps of:

measuring the density of said mixture,

sensing the temperature of said saturated solution to measure thedensity thereof,

combining said density measurements to provide an indication of saidsolids concentration, and

controlling the temperature of said mixture to maintain a predeterminedconcentratoin of said solids in said mixture.

7. The method of determining in a variable temperature environment theconcentration of a crystalline material in a mixture of said crystalsand a saturated solution of said material, the density of said mixturebeing a function of the density of said saturated solution and theconcentration of said crystals in said mixture, said method comprisingthe steps of:

measuring the density of said mixture,

sensing the temperature of said saturated solution to measure thedensity thereof, and

combining said density measurements to provide an indication of saidcrystalline concentration.

. 8. The method of regulating in a variable temperature environment theconcentration of a crystalline material in a mixture of said crystalsand a saturated solution of said material, the density of said mixturebeing a function of the density of said saturated solution and theconcentration of said crystals in said mixture, said method comprisingthe steps of:

measuring the density of said mixture,

sensing the temperature of said saturated solution to measure thedensity thereof,

combining said density measurements to provide an indication of saidcrystalline concentration, and controlling the temperature of saidmixture to maintain said crystalline concentration substantiallyconstant.

References Cited UNITED STATES PATENTS 2,349,482 5/ 1944 Welty '73532,362,661 11/1944 Peters et al. 73'452 2,898,466 8/1959 Lintz et al.250--83.3 2,919,351 12/1959 Swift 25083.6 2,968,727 1/1961 Otis 250-8363,060,313 10/1962 Ohmart et al. 250-833 RICHARD C. QUEISSER, PrimaryExaminer.

I. FISHER, D. SCHNEIDER, Assistant Examiners.

4. THE METHOD OF REGULATING THE CONCENTRATION OF SALT CRYSTALS IN AMIXTURE OF SAID CRYSTALS AND A SATURATED SOLUTION OF SAID SALT CRYSTALS,THE DENSITY OF SAID SATURATED SOLUTION INCREASING WITH THE TEMPERATUREOF SAID MIXTURE, SAID METHOD COMPRISING THE STEPS OF: PASSING A BEAM OFRADIATION THROUGH SAID MIXTURE, DETECTING SAID RADIATION PASSING THROUGHSAID MIXTURE TO PROVIDE A FIRST SIGNAL HAVING A FIRST COMPONENTPROPORTIONAL TO SAID SALT CRYSTALS CONCENTRATION AND A SECOND COMPONENTPROPORTIONAL TO SAID SATURATION SOLUTION DENSITY, SENSING THETEMPERATURE OF SAID MIXTURE TO PROVIDE A SECOND SIGNAL PROPORTIONAL TOTHE DENSITY OF SAID MIXTURE, AND COMBINING SAID SECOND SIGNAL WITH SAIDFIRST SIGNAL TO PROVIDE A THIRD SIGNAL PROPORTIONAL ONLY TO THECONCENTRATION OF SAID SALT CRYSTALS IN SAID MIXTURE, AND CONTROLLING THETEMPERATURE OF SAID MIXTURE IN ACCORDANCE WITH SAID THIRD SIGNAL TOMAINTAIN SAID CRYSTAL CONCENTRATION SUBSTANTIALLY CONSTANT.